1. Field of the Invention:
The present invention relates generally to hydrostatic extrusion methods and apparatus, and more particularly to a method and apparatus for hydrostatically extruding tubes including those tubes having external and internal peripheral surfaces which are irregular or asymmetric as seen in cross-section.
2. Description of the Prior Art:
Heretofore, among the various sealing methods and apparatus used in hydrostatic extrusion systems, one such method employs suitable means which effectively seals the tip portion of the billet while another method employs similar means which effectively seals the rear end portion of the billet. For example, the latter conventional method is illustrated in FIG. 1, such method being of the fixed mandrel type. A container 1 has a die 3, supported by means of a die-supporting body 2, disposed within the forward, hollow portion thereof, while a stem 4 is reciprocably disposed within the rear, hollow portion thereof. Within the conical cavity of the die 3 there is disposed the nose or tapered tip portion 6 of an annular billet 5, the diameter of the opening at the tip portion 6 being slightly less than that of a mandrel 7, the conical tip portion 8 of the mandrel 7 being inserted through the opening of the billet 5. The mandrel 7 is affixed to a mandrel ring 9 at the rear end thereof and the mandrel ring 9 is adapted to abut a mandrel supporting cylinder 10.
When extruding a tube having a circular cross-section, the mandrel 7 is advanced together with the mandrel ring 9 until the forward face of the mandrel ring 9 abuts the rear face of the mandrel supporting cylinder 10, whereupon forward progress of mandrel 7 is terminated. The tip or nose opening of the billet 5 is formed so as to have a diameter smaller than that of the mandrel 7, the tapered or conical nose portion 8 of the mandrel 7 being disposed through the aforesaid opening of the billet 5 so as to effect sealing therebetween. The forward, outer peripheral, conical surface of the billet 5 is likewise disposed within the hollow tapered or inner conical surface of the die 3 for similarly effecting sealing therebetween, such interfitting components serving to completely seal the engaged surfaces of the tip portion of the mandrel 7, billet 5 and the conical surface of the die cavity, respectively. It should be noted that the nose or tip portion of billet 5 has a sharp edge so as to effectively accomplish the desired sealing along the aforesaid tip portions as shown in FIG. 1, and thus it can be readily appreciated that the preparation and formulation of the nose portion of billet 5 requires an excessive number of man hours. In operation, when the stem 4 is advanced, the internal pressure within the container will of course be raised, and when such internal pressure reaches a predetermined value, the billet 5 will then be extruded as a tube through the annular interstice existing between die 3 and the tip portion of mandrel 7.
According to the aforenoted method, the pressure medium is disposed within cylinder 10 and about the inner and outer peripheral surfaces of billet 5 such that when the pressure medium is subjected to the high extrusion pressures, the medium will be forced into contact with the surfaces of the mandrel 7 and the billet 5 as well as the surfaces of the die 3 and cylinder 10, and the billet 5, whereupon the extrusion break-through pressure becomes relatively low. However, the pressure medium thus forced into contact with the various noted surfaces adversely affects the smoothness of the surfaces of the tube extruded. In addition, as the sealing is effected between the smallest diameter portion of the tapered or conical portion of the mandrel and the bearing portion of die 3, in order to prepare the nose portion of the billet, a great reduction-of-area-percentage is required for providing the sharp edge of the nose of the billet, and thus the cost of manufacture of the billets will be substantially increased even in instances where extrusion of a tube having a circular cross section is to be accomplished.
Furthermore, with respect to the aforesaid extrusion method, the extruded tube wall interstice defined between the small-diameter tip portion of the mandrel and the bearing portion of the die is such that the extrusion of a tube having an irregularly configured cross section is quite limited to the extent wherein only minor modifications from the circular cross section would be able to be attained. In other words, the extrusion of a tube having a wall of an irregular configuration is subject to other limitations in addition to those presented for a tube of circular cross-section. More particularly, as the sealing against undesired leakage of the pressure medium is effected due to the intimate contact between the inner conical surface of the die cavity and the forward, outer, conical peripheral surface of the billet and between the forward inner, conical, peripheral surface of the billet and the forward, outer, conical portion of the mandrel, the nose portion of the billet should be formed so as to have a configuration whose outer peripheral surface conforms to the inner conical surface of the die and whose inner peripheral surface conforms to the outer conical portion of the mandrel.
More specifically, in the instance that a tube having a wall of a substantially flat configuration in cross section is desired to be extruded according to the prior art method disclosed within FIG. 1, the outer and inner peripheral surfaces of the billet should have configurations which respectively conform to the configurations of the contacting surfaces of the die and mandrel, such as for example, elliptical configurations in cross section. Such requirements impose tremendous limitations upon the billet preparation apparatus designed to produce a billet having component portions of such desired configurations with accompanying difficulties being experienced in the technology and machining or the like required therefor. Thus, such billets are quite costly and are not readily adapted for industrial applications.
Furthermore, in the extrusion of tubular products according to hydrostatic extrusion methods, the outer peripheral configuration of the tube is governed by the opening within the die while the inner peripheral configuration thereof is dependent upon the sizing portion of the mandrel. The axial centering of the die and mandrel may be easily achieved wherein a tube having a circular cross section is desired, however, when a tube having a wall of an asymmetric or non-circular cross section is desired, such as for example, an ellipse, the relative positions of the die and mandrel should be adjusted prior to the extrusion and such relationship should be maintained constant throughout the period of pressurizing the container as well as the extrusion of the billet, such thereby presenting considerable additional difficulties to be resolved for such extrusion processes.
A still further disadvantage of such a conventional hydrostatic extrusion method, is that, as has been referred to earlier, the presence of the pressure medium along the surfaces of the die, the billet and the mandrel results in the poor exhibition of gloss characteristics upon the inner surface of the extruded tube due to the absence of direct contact between the billet and the mandrel due to the interdisposition of the pressure medium or lubricant therebetween. Still further, since the conical, inner peripheral surface of the billet remains in contact with the sizing or bearing portion of the mandrel, with the pressure medium being interposed therebetween, until the billet reaches a position at which its final inner and outer configurations are formed by the extrusion between the tip portion of the mandrel and the bearing portion of the die, then undesirable inner surface conditions often result upon the extruded tube, such as for example, the appearance of ripples or dimples thereon.